1. FIELD OF THE INVENTION
This invention relates to a new class of bioresistant surfactants and their use in formulating cutting fluids providing excellent corrosion inhibition, lubrication and most significantly, resistance to microbial degradation.
2. DESCRIPTION OF THE PRIOR ART
Modern metal working and forming processes normally require the use of cutting fluids whose function is to facilitate the machining operations by (1) cooling and (2) lubricating. The cooling function is accomplished by the ability of the fluid to carry off the heat generated by the frictional contact between the tool and the workpiece and/or any heat resulting from the plastic deformation of the work. Cooling aids tool life, preserves tool hardness and helps to maintain the dimensions of the machined parts. The second function is accomplished by the ability of the fluid to lubricate the tool-workpiece interface in order to reduce tool wear, frictional heat generation and power consumption. The presence of a fluid also serves to carry away debris from the work area.
In addition to the primary functions of cooling and lubricating, cutting fluids should protect the machined surfaces, tools and other equipment from rust and corrosion; should not themselves corrode, discolor or form deposits in or on the work area, and should not produce undesirable fumes or smoke. And, in those instances where skin contact is unavoidable, the cutting fluid compositions should be non-toxic and dermatologically safe.
Moreover, the cutting fluid composition itself should, ideally, have some stability against microorganisms. Bacteria and fungi frequently spoil soluble cutting fluids (especially oil in water emulsions) during machining operations. Not surprisingly, the cutting environment, which contains warm water and an available carbon source, provides a good medium for microorganism growth. Spoilage often manifests itself first as a foul smell. Furthermore, spoilage can cause color change, emulsion break, acidity increase, and sludge formation. Any of these signal a need to change the cutting fluid. Under severe conditions of use, changes due to biological fouling can be required every few days.
To remedy this situation, it is conventional in the art to add various "biocides" to cutting fluid compositions in order to control microbial growth and thus extend fluid life. However, biocides present their own problems. Some cause contact dermatitis, and others cause allergic dermatitis. Still others, through in situ modification, become suspect carcinogens. Government regulations and worker concern about biocide exposure further inhibit their use. Even in situations where the added biocides have no known adverse reaction to humans, biological fouling of the cutting fluid can still occur within a short time and lead to the serious problems of offensive odor, filter plugging, eventual loss of lubricative properties and corrosion of parts and machinery caused by acidic by-products generated by the microorganisms. Furthermore, if the cutting fluid is employed in the form of a stable emulsion with water, biological fouling can lead to the breaking of the emulsion. An ideal cutting fluid would thus contain no biocide while exhibiting bioresistance for prolonged periods of use.
Various types of cutting fluids are known in the art which embody certain of the desired characteristics delineated above. However, they also lack one or more of the same, such as corrosion resistance, biocidal activity and/or drill life performance, to the extent that they are inadequate in meeting the demands of practical industrial applications. Water, for example, is one of the most effective coolants available but can seldom be used as an effective cutting fluid in that it has little value as a lubricant and further it will promote rusting of ferrous work pieces. One method of combining the cooling properties of water with the lubricating properties of oil is through the use of soluble oils. These soluble oils are compounded so they will be able to form a stable emulsion with water. In such cases, the main component of the emulsion, i.e. water, provides effective cooling while the oil and other compounds impart the desirable properties of lubrication and corrosion resistance. Still, these emulsions often fall short of possessing all of the above-mentioned qualities required in a cutting fluid and, as such, remain somewhat inadequate for the rigors of practical metal working and machining operations.
As an example, U.S. Pat. No. 2,999,064 discloses a stable aqueous cutting fluid comprised of a mixture of boric acid, unsaturated fatty acids, such as those derived from oleic acid, and alkanolamines. While aqueous solutions are used as cutting fluids, their anti-corrosive character is found to be inadequate for practical use and they have the added disadvantage of foaming. Most importantly, the essential compounds of these cutting fluids do not themselves possess adequate bioresistance, hence requiring the use of exogenous biocidal compounds, which, of course, suffer from the drawbacks mentioned previously.
In DE-OS 1620447 and DE-OS 2007229, salts or condensation products of alkanolamines and ortho-boric acid are described as being foamless, water-hardening nonreactive rust inhibitors with fungistatic and bacteriostatic action. However, the corrosion inhibiting action of these compositions has also proven to be inadequate in actual practice.
U.S. Pat. No. 4,022,713 describes the reaction product of orthoboric acid and monoalkanolamines having one to three carbon atoms in the alkanol chain. These compounds possess bactericidal and fungicidal properties; however, they must be added separately to the cutting fluid composition for this purpose and, in any event, they manifest the inadequacies attendant exogenous biocides.
Lastly, soluble cutting oils, such as those exemplified by the current industrial standards, are known to degrade biologically, suggesting that oleic acid or other fatty acids, acting as a surfactant in these fluids, contributes most to spoilage by supplying food for microbial growth. As surfactants, sulfonates can replace carboxylates. However, sulfate-reducing bacteria convert sulfonates to hydrogen sulfide, a well known malodorant. In addition, the typical cutting fluid basestock--namely, mineral oil, contains normal paraffins which can also provide food to microorganisms.
Thus it has been discovered that highly branched surfactants when formulated into a cutting oil, resist microbial degradation without resort to externally added biocides while at the same time meeting all commercial standards of performance. Further, formulation of the cutting fluid using a branched basestock is found to enhance the bioresistance of the final fluid or emulsion.